1. Field of the Invention
The present invention relates to an image reading device favorably applicable to digital copiers.
2. Related Background Art
Generally, a digital copier includes an image reading device for reading an image depicted on a source document, an image processing circuit for processing, as needed, the image data read by the image reading device, and an output part for printing out the processed image. In order to read the two-dimensional image depicted on the source document, the image reading device includes an optical image-forming system accepting light from the source document and forming an image from the accepted light, a CCD sensor having a number of photoreceptors or pixels arranged in a horizontal scanning direction to detect light of the image formed by the optical image-forming system, and a scanner for scanning a reading-line position, read by the CCD sensor, on the source document in a vertical scanning direction. The scanner in the image reading device generally includes a pulse motor as a driving source, and a transmission mechanism (e.g., geared pulleys, and a timing belt) for transmitting the rotation of the pulse motor. A carriage or carrier included in the optical image-forming system is driven in the vertical scanning direction.
However, with such a scanner of the image reading device, the scanning of the reading line position in the vertical scanning direction by the CCD sensor involves microscopic misregistration of scanning speed due to a step-motion of the pulse motor and the mechanical engagement between the geared pulleys and the timing belt. Therefore, the image obtained by the image reading device is not an image faithfully copied the image depicted on the source document. With a view toward solving the above drawbacks, it is conceived that the scanning of the carriage can be flattened by applying an inertial force on the rotary axle of the pulse motor or by increasing an inertia of the carriage resulting from combining the carriage and magnet.
However, as the misregistration in scanning speed of the carriage in the vertical scanning direction tends to cause vibrations, a resonance phenomenon may occur at a certain scanning speed. Furthermore, a scanning speed of the carriage is set in accordance with a scaling factor of duplication (a ratio of image size on the document to image size printed out by the output part). It can be understood that the resonance phenomenon occurs at a certain scaling factor of duplication. Where the inertial force is applied on the rotary axle of the pulse motor and the inertia of the carriage is increased as mentioned above, it is possible merely to shift a scaling factor of duplication at which the resonance phenomenon occurs. Thus, the resonance phenomenon may still be encountered at a certain scaling factor of duplication or at a certain scanning speed. If such a resonance phenomenon occurs, quality of an image read by the image reading device will be degraded.
The present invention has been made in order to overcome the above drawbacks and has for its object to provide an image reading device capable of reading an image and outputting high-grade image data.
According to the present invention, an image reading device comprises: (1) a first optical image-forming system receiving light from an image on a readout surface and forming an image of the received light on an image-forming plane; (2) a CCD sensor arranged in the image-forming plane in parallel to a horizontal scanning direction to detect light of the image formed by said first optical image-forming system, said CCD sensor accumulating, in each pixel thereof, a charge in response to the amount of detected light; (3) means for scanning, in a vertical scanning direction perpendicular to the horizontal direction, a reading line of the image on the readout surface by said optical image-forming system and said CCD sensor; (4) means for reading out, as image data, the charge accumulated in each pixel of said CCD sensor; (5) means for detecting a scanning position of the scanning means; and (6) means for correcting the image data read out by said readout means based on the scanning position detected by said scanning-position detecting means.
According to this image reading device, the light from the image on the document to be read is focused through the optical image-forming system and detected by the CCD sensor. The image-reading line by the CCD sensor and the optical image-forming system is scanned in the vertical scanning direction by the scanning means and the charge accumulated in the CCD sensor is read by the readout means. The scanning position by the scanning means is detected by the scanning-position detecting means and the image data read by the readout means is corrected by the correcting means based on the scanning position detected by the scanning-position detecting means.
Preferably, the scan position detecting means of the image reading device according to the invention comprises: (1) a position-reading plate having a position-reading pattern depicted adjacent to said readout surface over a range of scanning by said scanning means; (2) a second optical image-forming system for use in position-reading, said second optical system receiving light from a portion of said position-reading pattern on an extension of the reading line scanned by said scanning means and forming an image thereof; and (3) means for detecting an amount of light of the image formed by said second optical image-forming system, said detecting means determining the scanning position scanned by said scanning means based on the detected amount of light. In this preferred arrangement, the light from a portion of the position-reading pattern on an extension of the reading line scanned by the scanning means is received by the second optical image-forming system. The detecting means detects an amount of light of the image formed by the second optical image-forming system, and the detecting means determines the scanning position scanned by the scanning means based on the detected amount of light.
The first and second optical image-forming systems may share a common configuration, and the detecting means may use pixels of the CCD sensor other than those used to detect the light of the image formed by the first optical image-forming system. In this case, as the CCD sensor and the first optical image-forming system for reading the image depicted on the source document is employed as the scanning-position detecting means, there is no need to provide an additional optical system. This prevents an image reader from being upsized.
The present invention will become more fully understood from the detailed description given hereinbelow and the accompanying drawings which are given by way of illustration only, and thus are not to be considered as limiting the present invention.
Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.